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SILURIAN

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Originally appearing in Volume V25, Page 111 of the 1911 Encyclopedia Britannica.
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SILURIAN , in See also:

geology, a See also:series of strata which is here under-stood to include those Palaeozoic rocks which See also:lie above the Ordovician and below the Devonian or Old Red See also:Sandstone, viz. the Llandoverian (Valentian of C. See also:Lapworth), Wenlockian and Ludlovian See also:groups of See also:Great See also:Britain with their See also:foreign equivalents. A word of caution is necessary, however, for in the See also:early See also:history of See also:British stratigraphy the exact delimitation of " Silurian" was the subject of a great controversy, and the See also:term has been used with such varying significance in See also:geological literature, that considerable confusion may arise unless the numerous interpretations of the See also:title are understood. The name " Silurian " was first introduced by See also:Sir R. I. See also:Murchison in 1835 for a series of rocks on the border counties of See also:England and See also:Wales—a region formerly inhabited by the See also:Silures. Murchison's Silurian em-braced not only the See also:rock groups indicated above, but others below them that were much older, even such as are now classed as See also:Cambrian. About the same See also:time A. See also:Sedgwick proposed the term Cambrian for a great See also:succession of rocks which includes much of Murchison's Silurian See also:system in its upper See also:part; hence arose that controversy which See also:left so lasting a See also:mark on British geology. In 185o A. d'See also:Orbigny suggested the name " Murchisonian " for what is here retained as the Silurian system. As a See also:solution of the difficulties of nomenclature, See also:Professor C. Lapworth in 1879 proposed the term Ordovician systems (q.v.) for those rocks which had been the See also:Lower Silurian of Murchison and the Upper Cambrian of Sedgwick.

An approximate correlation of the usages of the title " Silurian" is here given in tabulated See also:

form: The Silurian rocks are almost wholly of marine origin and include all the usual phases of sedimentation; shales and mudstones, marls and limestones, sandstones and grits are all represented in Great Britain and in most other countries where the Silurian is known. The See also:majority of the rocks were deposited in the comparatively shallow See also:waters of epicontinental seas, the graptolitic shales and sponge-bearing cherts being perhaps the representative's of the deeper waters. Locally, glauconitic limestones and ironstones (See also:Clinton beds) indicate See also:special conditions; while the See also:isolation and See also:desiccation of certain marine areas (New See also:York) towards the See also:close of the See also:period gave rise to beds of red sandstone, red marls, See also:gypsum and rock See also:salt. The See also:hydraulic See also:limestone (See also:Water See also:Lime) of New York was probably a brackish-water formation. In See also:Sweden and elsewhere some of the limestones and shales are distinctly bituminous. See also:Distribution.—In the preceding Ordovician period several well-marked marine provinces are indicated by the fossil contents of the rocks. At the beginning of Silurian time a See also:general transgression of the See also:sea—which had commenced at the close of the Ordovician—was in progress in the N. hemisphere (See also:Europe and the Appalachian region). This culminated at the time when the See also:Wenlock beds and their equivalents (Niagaran and See also:Oesel beds) were forming at the bottom of a great periarctic sea or shallow ocean. It is thus found that the same general characters prevail in the Silurian of Britain, N. See also:America, Scandinavia and the Baltic region, See also:Russian See also:Poland (See also:Podolia, See also:Kielce, See also:Galicia), the See also:Arctic regions, New See also:Siberia (Kotelny), Olenk See also:district, Waigatsch, N. Zembla, Tunguska, See also:Greenland, See also:Grinnell See also:Land and See also:China. The Bohemian region, comprising central Bohemia, Thuringia, See also:Fichtelgebirge, See also:Salzburg, See also:Pyrenees, See also:Languedoc, See also:Catalonia, See also:South See also:Spain, See also:Elba and See also:Sardinia, alone retained some of its marked individuality.

Later in the period a See also:

gradual withdrawal of the sea set in over the N. hemisphere, affecting the British See also:area (except See also:Devon), the left of the See also:Rhine, See also:Norway and the Baltic region, N. See also:Russia, Siberia and the Ural region, Spitzbergen, Greenland and the W. states of N: America. Thus the later Silurian conditions heralded those of the succeeding Devonian and Old Red Sandstone, and there is generally a gradual passage from one set of rocks to the other (Downtonian of Great Britain). The Silurian rocks may occur in close continuity with the upper Ordovician, as in S. Europe; or, as in the typical region, the See also:Llandovery beds may See also:rest unconformably upon older rocks; in N. America also there is a marked unconformity on this See also:horizon. A large part of N. America was apparently land during part of Silurian time; the lower members are found in the E. alone, while the Cayaguan See also:division is found to extend farther E. than the See also:middle or Niagaran division, but not so far W. The falls of See also:Niagara owe their existence to the presence of the hard See also:Lockport and See also:Guelph beds resting upon the softer See also:Rochester shales. Most of the essential See also:information as to the distribution of Silurian rocks will be found in a condensed form in the accompanying table and See also:map; but See also:attention may here be See also:drawn to the upper Silurian (Ludlovian) limestone of See also:Cornwallis See also:Island, the See also:mid-Silurian lime-See also:stone of Grinnell Land and the lower Silurian limestone of New Siberia. Limestones of lower and middle Silurian See also:age are found alsoin Timan, Tunguska and elsewhere in N. Russia.

Rocks of this system in S. America have been only superficially studied; they occur in the lower regions of the See also:

Amazon, where they See also:bear some resemblance to the See also:Medina and Clinton stages of N. America, and in See also:Bolivia and See also:Peru. Little is known of the Silurian rocks recorded from N. See also:Africa. Silurian See also:Life.—Our know-ledge of the life of this period is limited to the inhabitants of the seas and of the brackish waters of certain districts. The remains of marine organisms are abundant and varied. See also:Graptolites flourished as in the pre-ceding period, but the forms characteristic of the Ordovician gave See also:place early in the Silurian to the single-See also:axis type (Monograptidae) which prevailed until the close of the period (Rastrites, Monograptus, Retiolites and Cyrtograptus). As in the Ordovician rocks, the graptolites have been largely employed as zonal indicators. See also:Trilobites were important; the genera Calymene, Phacops and Encrinurus attained their maxi-mum development; Proetus, Bronteus, Cyphaspis, Arethusina may be mentioned from among many other genera. The ostracods Leperditia and See also:Bey- richia are very abundant locally. A feature of great See also:interest is the first See also:appearance of the remarkable Eurypterid crustacean Eurypterus, which occasionally reached the length of over a yard, and of the limulids, Neolimulus and Hemiaspis.

The cephalopods were the predominant molluscs, especially Orthoceras and various abbreviated or coiled orthoceras-like forms (Cyrtoceras, Phragmoceras, Trochoceras, Ascoceras) ; there was also a See also:

Nautilus, and an early form of goniatite has been recorded. Gasteropods include the genera Platyceras, Murchisonia and See also:Bellerophon; the pteropod Tentaculites is very abundant in certain beds. The pelecypods were not very important (Cypricardinia, Cardiola interrupta, C. cornucopiae). Next to the cephalopods in importance were the brachiopods: in the lower Silurian pentamerus-like forms still continued (P. Knightii, P. oblongus), but the See also:spire-bearing forms soon began to increase (Spirifer, Whitfieldia, Meristina, Atrypa). Other genera include Rhynchonella, Chonetes, Terebratula, Strophomena, Stricklandinia. The bryozoa, especially the bulky rock-See also:building forms, were less in See also:evidence than in the Ordovician. The echinoderms were well represented by the crinoids (Cyathocrinus, Crotalocrinus, Taxocrinus), some of which are found in a See also:state of beautiful preservation at See also:Dudley in England, Lockport (New York); Waldron (See also:Indiana) in N. America and also in Gothland in the Baltic. Cystids were abundant, but less so than in the Ordovician; blastoids made their first appearance. See also:Corals, mostly tabulate forms, flourished in great abundance in the clearer waters and frequently formed reefs (Favosites gothlandica, Halysites catenularia, Alveolites, Heliolites) ; tetracorallian forms include Stauria, Cyathophyllum, Cystiphyllum, Acervularia, Omphyma and the remarkable Goniophyllum. See also:Sponges were represented by Astylospongia, Aulocopium, &c.

The See also:

peculiar genera Silurian Period :/ Suggested distribution of Land & Water Abet de See also:Lapparent . toe shaded portions md%o,te tiviensin - wised See also:alien. rocks Moe been /send R. I. Murchison. A. Sedgwick. C. Lapworth. See also:American. A. de Lapparent. E. See also:Renevier.

Silurian. c = ro c a ' ai (Upper Silurian cr.) •i.0. m ° re, ^ •d a of some authors.) uv (=I 4 ti v Z. to ° b,o o 4 ~°ao-c .: 5 occ C C7 M Z . C5 (/1 a LLa se- e. — E c — Ordovician. = c •- o (Lower Silurian o E of some authors.) 0 CL ti E Um re: x '1~ a U M •~, .a U Cambrian. ro Upper. , Middle. - s:i Lower. U Graptolite England d c, a See also:

North America v`e ;~... 3i ~i .~ Zones and See also:Scotland. Scandinavian. Baltic Region. Bohemia.

.0 (New York). (Britain). Wales. U~ p d Monograptus Downton Downtonian Upper Cardiola Eurypterus See also:

Stage E2 of See also:Manlius leints.ardin- and and beds and upper beds. J. See also:Barrande. limestone. y •,: ens,s. See also:Ludlow Raeberry Cephalopod or a Rondout 3 ,o M. bohemicus. groups. See also:Castle Gothland lime- Limestones ° Water Lime. C S 2 •y 'a' beds. stone. with Cobleskill cephalopods.

limestone. t7 ° (n ° M. Vilssoni. Red sandstone. Upper Oesel See also:

Salina beds of d k; o y x Eis' E >' Lower cephalopod beds: o See also:Onondaga See also:ea u limestone. See also:dolomite with U eu ca ' ~ ° ZF tl Crinoid and See also:coral and c 4 rock salt d o c o° limestone. limestones. •~ and •^ ;; g c e ^ O gypsum. `°,c Lower Cardiola a a '"E shales and Mega- See also:harms p. :Q ~ a 8 harms limestones. C y r t o g r See also:apt u s Lower Oesel Crinoid oz S'j Guelph a BU a s o M. testis. Wenlock Riccarton, and See also:Blair, shales and lower beds: limestones. $ ^ dolomite. o Cyrtograptus Woolhope and brachiopod and dolomite N ° Z g Linnarssoni. groups. Straiton coral limestone and L' -o Lockport d ° beds. with sandstone. See also:marl.

E limestone. ¢ a y a ~ .2= Cvrtograptus ^.ty c7 b a Murchisoni, a Rochester ° shales. Z et . 5 .7•~ Clinton beds. '5 o MR cy boo i-7 N 3 4 o ^ y CO v rb o m '~/ . « q 8 n Rastrites Tarannon, See also:

Queensberry Rastrites shales Penlarnerus Stage Er of ,°._'-5 a -0 o o a .`r maximas. Llandovery, beds. and See also:Strickland- beds. J. Barrande. o~ `d Medina .~ o and inia marls. Graptolite a sandstone. Z a " ~' a u M. spinigerus. May Hilt Birkhill shales shales M m d ^ as `u ° a.a...

, 3 G ty o N groups. and with N See also:

Oneida V U 3•« M. gregarius. Graptolitic beds See also:diabase E See also:conglomerate. 2 N E Ey of the at the See also:base. us 0 °a Diplograptus See also:Girvan area. ° Shawangunk 0 3 Z ° c N'o toes iculosus. ma o. grit. a Li •E q c7 3 E Diplograplus p4° 00 ar uminalus. Receptaculites and Ischadites occur in the Silurian. See also:Foraminifera and See also:radiolaria also left their remains in the rocks. The most highly organized animals of the Silurian period were the fishes which had already made their appearance in the Ordovician rocks of See also:Colorado and Russia. The Silurian See also:fish include See also:selachians (Onchus, Thyestis), and the occurrence of remains of the obscure backboned See also:ostracoderms (placoderms) is particularly worthy of See also:notice (Pteraspis, Cephalaspis, Tremataspis, Cyathaspis, Thelodus, Lanarkia, Eukeraspis). Scorpions (Palaesphonus) have been found in See also:Lanark, Gothland and New York. Plant remains are very fully represented; land See also:plants have been recorded from the Harz and Kellerwald (H.

Potonie, 1901), and large silicified stems—up to 2 ft. in See also:

diameter—perhaps representing a gigantic seaweed (Nematophycus), have been found in Wales and in See also:Canada. Pachytheca is a small spherical See also:body often associated with Nematophycus. Girvanella is another obscure algal plant. As a natural result of the open See also:character of the great Silurian periarctic sea referred to above, there are many points of resemblance between the See also:fauna of the several regions of the N. hemisphere; this has been specially noticed in the community not only of genera but of See also:species between Britain, Sweden and the interior of N. America (See also:Iowa, See also:Wisconsin, See also:Illinois). ' Goniophyllum pyramidale is See also:common to Iowa and Gothland; Atrypa reticularis, Orthoceras annulatum and not a few others are common to Europe and N. America. An extremely interesting circumstance is the admixture of a periarctic and Bohemian fauna in the Australasian region. In a general sense the Silurian period was one of See also:comparative quiescence as regards crustal disturbances, and a relative sinking of the land was followed by a relative See also:elevation affecting wide areas in the N. hemisphere. See also:Local oscillations, such as those taking part in the formation of the Salina beds, &c., were naturally taking place, but the folding of the Scandinavian mountains and in the N. See also:highlands of Scotland continued throughout the period accompanied by a great amount of thrusting. Volcanic activity was quite subordinate in Silurian times; flows of diabase occurred at the commencement of the period in Bohemia, and evidence of See also:minor basaltic flows and tuffs is found at "I'ortworth in See also:Gloucestershire and at a few localities in N. America.

For further information, see articles on the CAMBRIAN, ORDOVICIAN, LLANDOVERY, \VENLOCII, LUDLOW Systems and Groups. (J. A.

End of Article: SILURIAN

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SILVA, ANTONIO JOSE DA (1705–1739)

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